The present invention relates to a getter device producing a reduced amount of methane in the residual gases of a vacuum tube, in particular an electron tube.
Getter devices are well known in the art. They are used for the production and maintenance of very low pressures within vacuum enclosures, such as, electron tubes and other electronic devices. They find particularly wide-spread use in cathode ray tubes used for the visual representation of images or other information. There are many factors which can affect the life of a cathode ray tube (CRT). Whilst the total residual gas pressure in the CRT must be maintained as low as possible, it is also important to have for instance a low partial pressure of oxygen containing gases. These latter are well known to poison the cathodes. It is especially important to have as low a partial pressure as possible of methane as the getter film, usually barium, has a negligible pumping speed for this gas. Although this gas can in effect be pumped by silicate binders used in the preparation of the phosphor layers on the screen during tube operation, its presence is very dangerous as it can cause ion bombardment of the cathodes or lead to their contamination with carbon deposits. The presence of methane within the CRT is apparently caused by the getter device itself. The main cause of the presence of methane would appear to be due to chemical reactions with barium carbide. The barium carbide being formed either from carbon impurities present in the original getter device or from subsequent reaction of the barium film with carbon containing residual gases in the CRT such as carbon monoxide and carbon dioxide. Whatever the reason for the presence of methane after the evaporation of a getter device within a vacuum vessel it is necessary to reduce its partial pressure to as low a value as possible.
Some tube manufacturers tolerate a small amount of methane as it tends to produce a reducing atmosphere which is regarded as beneficial for cathode activation, however, it can still cause ion bombardment of the cathodes with their eventual destruction, or contaminate them with carbon deposits to which some types of cathode are particularly sensitive.
It is therefore an object of the present invention to provide a getter device free from one or more of the defects of prior art getter devices.
Another object of the present invention is to provide a getter device which produces a reduced partial pressure of methane within a vacuum enclosure.
A further object of the present invention is to provide a getter device which provides a suitable gas ambient within an electron device for the activation of the cathode.